990 resultados para Beam angle selection
Resumo:
Post-irradiation studies have been carried out to elucidate the effects of electron beam irradiation on the structural, optical, dielectric, and thermal properties of high-density polyethylene (HDPE) films. The experimental results showed that both the optical band gap and activation energy of HDPE films decreases with an increase in the doses of electron radiation. The electrical measurements showed that dielectric constant and the ac conductivity of HDPE increases with an increase in the dose of electron radiation. The thermal analysis carried out using DSC and TGA revealed that the melting temperature, degree of crystallinity, and thermal stability of the HDPE films increased, obviously, due to the predominant cross-linking reaction following high doses of electron irradiation.
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Lamination-dependent shear corrective terms in the analysis of bending of laminated plates are derived from a priori assumed linear thicknesswise distributions for gradients of transverse shear stresses by using CLPT inplane stresses in the two in-plane equilibrium equations of elasticity in each ply. In the development of a general model for angle-ply laminated plates, special cases like cylindrical bending of laminates in either direction, symmetric laminates, cross-ply laminates, antisymmetric angle-ply laminates, homogeneous plates are taken into consideration. Adding these corrective terms to the assumed displacements in (i) Classical Laminate Plate Theory (CLPT) and (ii) Classical Laminate Shear Deformation Theory (CLSDT), two new refined lamination-dependent shear deformation models are developed. Closed form solutions from these models are obtained for antisymmetric angle-ply laminates under sinusoidal load for a type of simply supported boundary conditions. Results obtained from the present models and also from Ren's model (1987) are compared with each other.
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Lamination-dependent shear corrective terms in the analysis of flexure of laminates are derived from a priori assumed linear thicknesswise distributions for gradients of transverse shear stresses and using them in the two in-plane equilibrium equations of elasticity in each ply. Adding these corrective terms to (i) Classical Laminate Plate Theory (CLPT) displacements and (ii) Classical Laminate Shear Deformation Theory (CLSDT) displacements, four new refined lamination-dependent shear deformation models for angle-ply laminates are developed. Performance of these models is evaluated by comparing the results from these models with those from exact elasticity solutions for antisymmetric 2-ply laminates and for 4-ply [15/-15](s) laminates. In general, the model with shear corrective terms based on CLPT and added to CLSDT displacements is sufficient and predicts good estimates, both qualitatively and quantitatively, for all displacements and stresses.
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We report the structural and optical properties of a-plane GaN film grown on r-plane sapphire substrate by plasma-assisted molecular beam epitaxy. High resolution X-ray diffraction was used to determine the out-of-plane and in-plane epitaxial relation of a-plane GaN to r-plane sapphire. Low-temperature photoluminescence emission was found to be dominated by basal stacking faults along with near-band emission. Raman spectroscopy shows that the a-GaN film is of reasonably good quality and compressively strained. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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Nicotinate-N-oxide and isonicotinate-N-oxide have been employed to synthesize four heterometallic metallamacrocycles (dppf)(2)Pd-2(nicotinate-N-oxide)(2)](OTf)(2) (1), (dppf)(2)Pt-2(nicotinate-N-oxide)(2)](OTf)(2) (2), (dppf) 2Pd2(isonicotinate-N-oxide)(2)](OTf)(2) (3) and (dppf)(2)Pt-2(isonicotinate-N-oxide)(2)](OTf)(2) (4). The complexes represent the first examples of metallamacrocycles driven by solely Pd(II)/Pt(II)-O coordination using carboxylate-N-oxide donor. All the complexes 1-4 are characterized by IR, UV-Vis, multinuclear NMR spectroscopic and ESI-MS studies. The molecular structures of the complexes 1 and 3 are unambiguously determined by single crystal X-ray diffraction analysis. Despite the possibility of formation of several linkage isomers due to ambidentate nature of the donors, exclusive formation of 2 + 2] self-assembled single isomeric metallamacrocycle in each case is interesting observation. (C) 2011 Elsevier B.V. All rights reserved.
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NMR spectra of liquid crystalline phases and the molecules dissolved therein, spinning at and near the magic angle provide information on the director dynamics and the order parameter. The studies on the dynamics of the liquid crystal director for sample spinning near magic angle in mesophases with positive and negative diamagnetic susceptibility anisotropies (Delta chi) and their mixtures with near-zero macroscopic diamagnetic susceptibility anisotropies have been reported. In systems with weakly positive Delta chi, the director has been observed to switch from an orientation parallel to the spinning axis at low rotational speeds to one perpendicular to the spinning axis at high rotational speeds, when the angle theta, the axis of rotation makes with the magnetic field is smaller than the magic angle theta(m). For systems with a small negative Delta chi, similar director behaviour has been observed for theta greater than theta(m). At magic angle, the spectra under slow spinning speeds exhibit a centre band and side bands at integral values of the spinning speeds. The intensities of the spinning side bands have been shown to contain information on the sign and the magnitude of the order parameter(s). The results are discussed with illustrative examples. Results on the orientation of the chemical shielding tensor obtained from a combination of the NMR studies in the solid and the liquid crystalline states, have been described.
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Optical and structural properties of reactive ion beam sputter deposited CeO2 films as a function of oxygen partial pressures (P-O2) and substrate temperatures (T-s) have been investigated. The films deposited at ambient temperature with P-O2 of 0.01 Pa have shown a refractive index of 2.36 which increased to 2.44 at 400 degrees C. Refractive index and extinction coefficient are sensitive up to a T-s of similar to 200 degrees C. Raman spectroscopy and X-ray diffraction (XRD) have been used to characterise the structural properties. A preferential orientation of (220) was observed up to a T-s of 200 degrees C and it changed to (200) at 400 degrees C: and above. Raman line broadening, peak shift and XRD broadening indicate the formation of nanocrystalline phase for the films deposited up to a substrate temperature of 300 degrees C. However, crystallinity of the films were better for T-s values above 300 degrees C. In general both optical and structural properties were unusual compared to the films deposited by conventional electron beam evaporation, but were similar in some aspects to those deposited by ion-assisted deposition. Apart from thermal effects, this behavior is also attributed to the bombardment of backscattered ions/neutrals on the growing film as well as the higher kinetic energy of the condensing species, together resulting in increased packing density. (C) 1997 Elsevier Science S.A.
Suboptimal Midcourse Guidance of Interceptors for High-Speed Targets with Alignment Angle Constraint
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Using the recently developed computationally efficient model predictive static programming and a closely related model predictive spread control concept, two nonlinear suboptimal midcourse guidance laws are presented in this paper for interceptors engaging against incoming high-speed ballistic missiles. The guidance laws are primarily based on nonlinear optimal control theory, and hence imbed effective trajectory optimization concepts into the guidance laws. Apart from being energy efficient by minimizing the control usage throughout the trajectory (minimum control usage leads to minimum turning, and hence leads to minimum induced drag), both of these laws enforce desired alignment constraints in both elevation and azimuth in a hard-constraint sense. This good alignment during midcourse is expected to enhance the effectiveness of the terminal guidance substantially. Both point mass as well as six-degree-of-freedom simulation results (with a realistic inner-loop autopilot based on dynamic inversion) are presented in this paper, which clearly shows the effectiveness of the proposed guidance laws. It has also been observed that, even with different perturbations of missile parameters, the performance of guidance is satisfactory. A comparison study, with the vector explicit guidance scheme proposed earlier in the literature, also shows that the newly proposed model-predictive-static-programming-based and model-predictive-spread-control-based guidance schemes lead to lesser lateral acceleration demand and lesser velocity loss during engagement.
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Four types of cationic collectors were tested and evaluated for their performance in the reverse flotation of silica from the spiral preconcentrate of Kudremukh iron ore. A stagewise flotation was conducted by adding the reagent in three stages. Starch was used to depress hematite. Silica flotation was found to be very sensitive to the amount of cationic reagent added. The performance of the reagents was evaluated based on the percentage of silica and iron in the concentrate and percent recovery of iron obtained in the concentrate. Tests of significance, namely, t-test and F-test were performed to select the best two reagents for further investigations.
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Measurements of small-angle neutron scattering (SANS) cross sections from different mixed micelles composed of CTAB and Br-, n-C16H33N+Me2-(CH2)(m)N+Me2-n-C16H33, Br- (16-m-16, 2Br(-), where m = 3, 5, and 10), in aqueous media (D2O) are reported. The data have been analyzed using the Hayter and Penfold model for macroion solution to compute the interparticle structure factor S(Q) taking into account the screened Coulomb interactions between the micelles. The aggregate composition matches with that predicted from an ideal mixing model. The SANS analysis further indicates that the extent of aggregate growth and the Variations of shapes of the mixed micelles could be modulated by the amount of dimeric surfactant present in these mixtures. With the spacer chain length m less than or equal to 4 in the dimeric surfactant, the propensity of micellar growth is particularly pronounced. The effect of the variation of the temperature for the mixed micellar system (23.1 mol % of 16-3-16, 2Br(-)) was also examined. The systemic microviscosities that the mixed micellar aggregates offer to a solubilized, extrinsic fluorescence probe, 1,6-diphenyl-1,3,5-hexatriene, were determined. The variation of the microviscosities of the mixed micelles as a function of percentages of the dimeric surfactants could be explained in terms of conformational variations and progressive looping of the spacer chain of dimeric surfactants in mixed micellar aggregates with increasing m values.
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The subsurface microhardness mapping technique of Chaudhri was utilized to determine the shape, size and distribution of plastic strain underneath conical indenters of varying semi-apex angles, alpha (55 degrees, 65 degrees and 75 degrees). Results show that the elastic-plastic boundary under the indenters is elliptical in nature, contradicting the expanding cavity model, and the ellipticity increases with alpha. The maximum plastic strain immediately under the indenter was found to decrease with increasing alpha. Complementary finite-element analysis was conducted to examine the ability of simulations to capture the experimental observations. A comparison of computational and experimental results indicates that the plastic strain distributions as well as the maximum strains immediately beneath the indenter do not match, suggesting that simulation of sharp indentation requires further detailed studies for complete comprehension. Representative strains, epsilon(r), evaluated as the volume-average strains within the elastic-plastic boundary, decrease with increasing alpha and are in agreement with those estimated by using the dimensional analysis. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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Dimeric or gemini surfactants consist of two hydrophobic chains and two hydrophilic head groups covalently connected by a hydrophobic or hydrophilic spacer. This paper reports the small-angle neutron scattering (SANS) measurements from aqueous micellar solutions of two different recently developed types of dimeric surfactants: (i) bis-anionic C16H33PO4--(CH2)(m)-PO4-C16H33,2Na(+) dimeric surfactants composed of phosphate head groups and a hydrophobic polymethylene spacer, referred to as 16-m-16,2Na(+), for spacer lengths m = 2, 4, 6, and 10, (ii) bis-cationic C16H33N+(CH3)(2)-CH2-(CH2-O-CH2)(p)-CH2-N+ (CH3)(2)C16H33,2Br(-) dimeric surfactants composed of dimethylammonium head groups and a wettable polyethylene oxide spacer, referred to as 16-CH2-p-CH2-16,2Br(-), for spacer lengths p = 1 - 3. The micellar structures of these surfactants are compared with the earlier studied bis-cationic C16H33N+ (CH3)(2)-(CH2)(m)-N+ (CH3)(2)C16H33,2Br(-) dimeric surfactants composed of dimethylammonium head groups and a hydrophobic polymethylene spacer, referred to as 16-m-16,2Br(-). It is found that 16-m-16,2Na(+), similar to 16-m-16,2Br(-), form various micellar structures depending on the spacer length. Micelles an disklike for rn = 2, rodlike for m = 4, and prolate ellipsoidal fur m = 6 and 10. The micelles of 16-CH2-p-CH2-16,2Br(-) are prolate ellipsoidal for all the values of p = 1 - 3. It is also found that micelles of 16-m-16,2Na(+) and 16-CH2-p-CH2-16,2Br(-) are large in comparison to those of 16-in-16,2Br(-) for similar spacer lengths. This is connected with the fact that both in 16-m-16,2Na(+) and 16-CH2-p-CH2-16,2Br(-), the head group or the spacer is more hydrated as compared to that in the 16-m-16,2Br(-). An increase in the hydration of the spacer or the head group increases the screening of the Coulomb repulsion between the charged head groups. This effect has been found to be more pronounced in the dimeric surfactants having wettable spacers. [S1063-651X(99)00303-7].
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Dynamics of the aircraft configuration considered in this paper show a unique characteristic in that there are no stable attractors in the entire high angle-of-attack flight envelope. As a result, once the aircraft has departed from the normal flight regime, no standard technique can be applied to recover the aircraft. In this paper, using feedback linearization technique, a nonlinear controller is designed at high angles of attack, which is engaged after the aircraft departs from normal flight regime. This controller stabilizes the aircraft into a stable spin. Then a set of synthetic pilot inputs is applied to cause an automatic transition from the spin equilibrium to low angles of attack where the second controller is connected. This controller is a normal gain-scheduled controller designed to have a large domain of attraction at low angles of attack. It traps the aircraft into a low angle-of-attack level flight. This entire concept of recovery has been verified using six-degrees-of-freedom nonlinear simulation. Feedback linearization technique used to design a controller ensures internal stability only if the nonlinear plant has stable zero dynamics. Because zero dynamics depend on the selection of outputs, a new method of choosing outputs is described to obtain a plant that has stable zero dynamics. Certain important aspects pertaining to the implementation of a feedback linearization-based controller are also discussed.
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Antenna selection (AS) provides most of the benefits of multiple-antenna systems at drastically reduced hardware costs. In receive AS, the receiver connects a dynamically selected subset of N available antennas to the L available RF chains. The "best" subset to be used for data reception is determined by means of channel estimates acquired using training sequences. Due to the nature of AS, the channel estimates at different antennas are obtained from different transmissions of the pilot sequence, and are, thus, outdated by different amounts in a time-varying channel. We show that a linear weighting of the estimates is optimum for the subset selection process, where the weights are related to the temporal correlation of the channel variations. When L is not an integer divisor of N, we highlight a new issue of "training voids", in which the last pilot transmission is not fully exploited by the receiver. We present a "void-filling" method for fully exploiting these voids, which essentially provides more accurate training for some antennas, and derive the optimal subset selection rule for any void-filling method. We also derive new closed-form equations for the performance of receive AS with optimal subset selection.